304 ANIMAL LIFE 



ence increases with time and with distance so long as the 

 separation is maintained. Hence new species and new 

 fauna or aggregations of species are produced wherever 

 free diffusion is checked by any kind of barrier. 



161. Faunal areas of the sea. In like manner, we may 

 divide the oceans into faunal areas or zones, according to 

 the distribution of its animals. For this purpose the fishes 

 probably furnish the best indications, although results very 

 similar are obtained when we consider the mollusks or the 

 Crustacea. The fresh-water fishes are not considered here, 

 as in regard to their faunal areas they agree with the land 

 animals of the same regions. Perhaps the most important 

 basis for primary divisions is found in the separation from 

 the localized shore-fishes of the cosmopolitan pelagic species, 

 and the scarcely less widely distributed bassalian species or 

 fishes of the deep sea. 



The pelagic fishes are those which inhabit the open sea, 

 swimming near the surface, and often in great' schools. 

 Such forms are mainly confined to the warmer waters. 

 They are for the most part predatory fishes, strong swim- 

 mers, and many of the species are found in all warm seas. 

 Most species have special homing waters, to which they 

 repair in the spawning season. Often there will be special 

 regions to which they never resort, either for breeding or 

 for food. At other times a certain species will appear in 

 numbers in regions where it has hitherto been unknown. 

 For example, the frigate-mackerel (Auxis thazard), homing 

 in the East Indies and the Mediterranean, appeared in 

 great numbers in 1880 off the coast of New England. Typ- 

 ical pelagic fishes are the mackerel, tunny, dolphin, flying- 

 fish, opah, and some species of shark. This group shades 

 off by degrees into the ordinary shore-fish, some being partly 

 pelagic, venturing out for short distances, and some are 

 pelagic for part of the year only. To the free-swimming 

 forms of classes of animals lower than fishes, found in the 

 open ocean, the name Plankton is applied. 



